You state that as if it were categorically true and that just isn't so. There were plenty of people jumping onto and off of the "Michael Jordan is the greatest player in the game" before they won their first championship.
As for Lebron James, yes there have been a lot of people labeling him as the best ever and that started while he was still in high school. And yes, he has failed to deliver on that promise. However, that's really at the heart of this discussion. His career is not yet over and he's not the first GREAT player to have come up short.
His 4th quarter disappearances against Dallas are certainly a bleak mark on his record but he has stepped up in the past and I'm quite certain he will again in the future.
that's just it Steve, in the biggest pressure moments, he HASN'T. Pressure on the Team USA in the gold medal game, he was playing hot potato and by all accounts looked like a scared bitch, literally biting his fingers during tense moments of the game. Going up against Boston with HCA and being the favorite, he literally quit on the series and now in the Finals, with all the talent in the world, when the going got tough, he got going.
He plays INCREDIBLY well when there's no pressure on him. Let's look at his great playoff moments... the RIDICULOUS 29 point burst against the Pistons in Game 5 and winning that series in 6... ALL the pressure was on Detroit. The Cavs were a heavy underdog, the 3 seed playing the reigning superpower in the East at the time. if he lost there it was okay because no one expected him to get that far. However, once he got into the Finals that year and people were comparing him to MJ, what happened? He went completely into the tank.
fast forward to 2008, when he had his incredible Game 7 against Boston, scoring 45 in a loss. Well, that Boston team was favored to win the title the second they traded for KG and Allen, won 66 games (19 more than the Cavs)... no one expected anything from him here either and he played carefree because there would be no consequences if he lost.
And that's pretty much it for his greatness in the playoffs. Since then (and before) when faced with pressure, the guy has completely crumbled, and I mean COMPLETELY FOLDED to the point of looking like he flat out quit... sometimes with all the talent in the world (Olympics and 2011 Finals), sometimes without (2010 v. Boston).
He's really yet to show he can handle pressure at all. Fair or not, he bailed on leading a team so he could, in his own words, "not have to feel like he faced pressure to be the man every night". That is a GLARING admission and point blank evidence that dude doesn't want the responsibility of being the absolute man.